Twig Beetle Research Articles

Genetic differentiation and spatial structure of Geosmithia morbida, the causal agent of thousand cankers disease in black walnut (Juglans nigra)

The main objectives of this study were to evaluate genetic composition of Geosmithia morbida populations in the native range of black walnut and provide a better understanding regarding demography of the pathogen. The fungus G. morbida, and the walnut twig beetle, Pityophthorus juglandis, have been associated with a disease complex of black walnut (Juglans nigra) known as thousand cankers disease (TCD). The disease is manifested as branch dieback and canopy loss, eventually resulting in tree death. In 2010, the disease was detected in black walnut in Tennessee, and subsequently in Virginia and Pennsylvania in 2011 and North Carolina in 2012. These were the first incidences of TCD east of Colorado, where the disease has been established for more than a decade on indigenous walnut species. A genetic diversity and population structure study of 62 G. morbida isolates from Tennessee, Pennsylvania, North Carolina and Oregon was completed using 15 polymorphic microsatellite loci. The results revealed high haploid genetic diversity among seven G. morbida populations with evidence of gene flow, and significant differentiation among two identified genetic clusters. There was a significant correlation between geographic and genetic distance. Understanding the genetic composition and demography of G. morbida can provide valuable insight into recognizing factors affecting the persistence and spread of an invasive pathogen, disease progression, and future infestation predictions. Overall, these data support the hypotheses of two separate, highly diverse pathogen introductions into the native range of black walnut.

The Effect of Temperature on Survival ofPityophthorus juglandis(Coleoptera: Curculionidae)

The walnut twig beetle (Pityophthorus juglandis Blackman) vectors Geosmithia morbida, the causal agent of thousand cankers disease in Juglans, and is particularly damaging to Juglans nigra L. (black walnut). Native hosts of P. juglandis are distributed in the southwestern United States where winter temperatures tend to be higher than those found within the native range of black walnut. To better understand temperature effects on survival of P. juglandis, we initiated studies to determine: 1) seasonal variations in cold tolerance, as measured by the supercooling point (SCP), and 2) upper and lower lethal temperatures (LT). In the lower LT study, Xyleborinus saxeseni (Ratzeberg) was tested for comparison. Insects were either exposed to increasing or decreasing temperatures and then checked for survival. Upper and lower LTs were estimated using a logistic model. For the SCP study, data were grouped into seasons. Seasonal mean SCPs were highest in summer (-15.4°C) and lowest in fall (-18.1°C). The upper lethal limit estimations required to kill 99% of the population (LT99) for adults and larvae were 52.7 and 48.1°C, respectively, and lower limit LT99 estimations for adults and larvae were -18.1 and -18.7°C, respectively. The lower median LT (LT50) of X. saxeseni was -24.7°C. These studies, as well as beetle survival in infested Colorado trees where temperatures reached -29°C in February 2011, suggest P. juglandis could survive the winter in much of the native range of black walnut, but may be limited in trees where temperatures regularly exceed the lower LT.

Susceptibility of Walnut and Hickory Species to Geosmithia morbida.

Thousand cankers disease (TCD) of walnut is a result of feeding in the phloem by the walnut twig beetle (WTB), Pityophthorus juglandis, and subsequent canker formation caused by Geosmithia morbida around galleries. TCD has caused extensive morbidity and mortality to Juglans nigra in the western United States and, in 2010, was discovered in the eastern United States, where the tree is a highly valuable timber resource. WTB and G. morbida also have been found in J. regia orchards throughout major production areas in California, and the numbers of damaged trees are increasing. We tested the susceptibility of walnut and hickory species to G. morbida in greenhouse and field studies. Carya illinoinensis, C. aquatica, and C. ovata were immune. All walnut species tested, including J. ailantifolia, J. californica, J. cinerea, J. hindsii, J. major, J. mandshurica, J. microcarpa, J. nigra, and J. regia, developed cankers following inoculation with G. morbida. J. nigra was the most susceptible, whereas J. major, a native host of the WTB and, presumably, G. morbida, had smaller and more superficial cankers. Canker formation differed among maternal half-sibling families of J. nigra and J. cinerea, indicating genetic variability in resistance to G. morbida. Our inoculation studies with G. morbida have corroborated many of the field observations on susceptibility of walnut and hickory species to TCD, although the ability of the WTB to successfully attack and breed in walnut is also an important component in TCD resistance.

Field Response of the Twig Beetle,Pityophthorus pubescens,to the Aggregation Pheromone, (E)-(+)-pityol, is not Inhibited by (E)-(-)-pityol, and Evidence of Monogyny

Field studies were conducted to determine the flight response of the twig beetle Pityophthorus pubescens (Marsham) (Coleoptera: Curculionidae: Scolytinae) to the potential aggregation pheromone component, (E)-(+)-pityol, and its racemic form, (E)-(±)-pityol, in different Pinus spp. L. (Pinales: Pinaceae) stands in the Basque Country (Northern Spain). Both (E)-(+)-pityol and (E)-(±)-pityol equally attracted more males than females, and they were all significantly different from the blank controls. Data about P. pubescens gallery systems in naturally infested P. radiata D. Don branches are also provided. The presence of one male and one female was the most common gallery habitation found in debarked twigs. Current results suggest that P. pubescens may be monogynous and females might mediate the aggregation of mating partners.

Electrophysiological and behavioural responses of Pityophthorus pubescens (Coleoptera: Scolytinae) to (E,E)‐α‐farnesene, (R)‐(+)‐limonene and (S)‐(−)‐verbenone in Pinus radiata (Pinaceae) stands in northern Spain

Some twig beetles in the genus Pityophthorus (Coleoptera: Scolytinae) may vector pitch canker disease Fusarium circinatum (Niremberg & O'Donnell) of Pinus spp. (Pinaceae). Because Pityophthorus pubescens (Marsh.) has been found to be associated with F. circinatum in the Basque Country (northern Spain), various experiments were conducted to assess the beetle's behavioural responses to (E, E)-α-farnesene, (R)-(+)-limonene and (S)-(-)-verbenone to develop a potential inhibitor to host attraction. These experiments comprise electroantennographic and double-choice olfactometer tests, as well as field assays in Pinus radiata D. Don stands. Both sexes of P. pubescens showed similar electroantennographic responses to different doses (from 1 ng to 1 µg in decadic steps) of each individual compound, with depolarisations to (S)-(-)-verbenone (100 ng) being similar to those of the aggregation pheromone (+)-trans-pityol. In olfactometer assays, both sexes were significantly attracted to (+)-trans-pityol, but the attraction was reduced when increasing amounts of the chemicals were added to the pheromone. Particularly relevant was the repellent effect induced by (S)-(-)-verbenone at 1 ng dose and higher. In the field, (E, E)-α-farnesene, (R)-(+)-limonene and (S)-(-)-verbenone reduced significantly the number of beetles attracted to (+)-trans-pityol and racemic trans-pityol, with (S)-(-)-verbenone being the most effective. (S)-(-)-Verbenone showed an interesting potential for use in the protection of P. radiata stands. A potentially effective strategy, which could be implemented in further, more in-depth studies, could involve the use of this semiochemical as repellent and (+)-trans-pityol-baited traps as attractant in a 'push-pull' strategy.

Disruption of trans‐pityol‐mediated attraction by racemic trans‐conophthorin in twig beetle Pityophthorus pubescens

AbstractTwig beetle Pityophthorus pubescens (Coleoptera: Curculionidae: Scolytinae) has been previously associated with the Fusarium circinatum (Hypocreales: Nectriaceae), the pathogen causing pitch canker disease, in P. radiata stands of the Basque Country (Northern Spain). Laboratory and field studies were conducted to evaluate the response of the insect to the racemic mixture of the spiroacetal trans‐7‐methyl‐1,6‐dioxaspiro[4.5]decane, also known as conophthorin. In walking bioassays, addition of 10 and 100 ng of racemic trans‐conophthorin to 1 ng of (±)‐trans‐pityol elicited a negative response in males, whereas females did not show any significant preference. Catches of males in attractant‐baited traps were strongly reduced by racemic trans‐conophthorin at all release rates tested (0.5, 1.0, 1.5 and 2.0 mg/day). In contrast, catches of females were not affected by any of the (±)‐trans‐conophthorin release rate.

An analysis of the larval instars of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), in northern California black walnut, Juglans hindsii, and a new host record for Hylocurus hirtellus

By measuring and analyzing larval head capsule widths, we determined that a northern California population of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), has three larval instars. We also developed rules to classify P. juglandis larval instars. Overlap in the ranges of widths among consecutive instars was addressed by fitting a distribution model to the data and determining limits for head capsule width classes and probabilities of instar misclassification. Growth ratios for laboratory and field samples of P. juglandis were consistent with the Brooks-Dyar rule and with results in the literature for other bark and ambrosia beetle species. As a consequence of branch dissection to retrieve larvae of P. juglandis, we established that northern California black walnut, Juglans hindsii (Jeps.) Jeps. ex R.E. Sm., is a host for the xylophagous “California hardwood bark beetle,” Hylocurus hirtellus (LeConte) (Coleoptera: Scolytidae), and that H. hirtellus is associated with P. juglandis beneath the bark of small diameter branches. It appears that in northern California, univoltine H. hirtellus is active in flight from early March through early May, with the majority of the adults dispersing in April. The females are the colonizing sex.

Recent collecting reveals new state records and geographic extremes in the distribution of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), in the United States

Recent collecting reveals new state records and geographic extremes in the distribution of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), in the United States

Diurnal flight response of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), to pheromone-baited traps in two northern California walnut habitats

Abstract The diurnal flight response of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), was assessed during two seasonal periods at two sites in northern California. Males and females flew primarily at dusk in response to aggregation pheromone-baited traps during late June/early July, and the percentage of beetles that flew between sunrise and late afternoon was positively correlated with the temperature at 6 AM of that day. Between late August and early November, the diurnal flight pattern was more varied and generally bimodal, though approx. 42 to 55% of the beetles still flew during the dusk period. During either period of the season, there was very little evidence that any flight occurred during the night. Between late August and late September, diurnal flight of both sexes during two-hour intervals was associated with temperature in a Gaussian manner with a threshold at 17–18 °C (62.6–64.4 °F) and peak activity at 23–24 °C (73.4–75.2 °F). The proportion of females i...

Effect of Geosmithia morbida Isolate and Temperature on Canker Development in Black Walnut

Thousand cankers disease of black walnut (Juglans nigra) is the result of aggressive feeding by the walnut twig beetle (Pityophthorus juglandis) and extensive cankering around beetle galleries caused by the fungus Geosmithia morbida. We developed a consistent, reproducible inoculation technique to screen black walnut trees for their reaction to canker development following inoculation with G. morbida. Canker areas in one-year-old trees were not affected by the location on the stem that inoculations were made. Differences in aggressiveness of G. morbida isolates, representing different rDNA ITS haplotype groups, to black walnut were observed in some experiments. However, these differences were small and evidence indicates that a single, highly aggressive haplotype is not responsible for the current TCD epidemic. Cankers formed in black walnut at all temperatures tested, but they were consistently smaller at 32/20°C day/night temperatures compared to 25/20°C. Although G. morbida is thermotolerant, higher temperatures may not enhance canker development. Accepted for publication 1 May 2012. Published 18 June 2012.

Initial Assessment of Thousand Cankers Disease on Black Walnut, Juglans nigra, in Eastern Tennessee

In 2010, thousand cankers disease (TCD) was documented in Tennessee, representing the first confirmation of this disease in the native range of black walnut and the first known incidence of TCD east of Colorado. Tennessee Department of Agriculture personnel conducted surveys to determine the extent of TCD in counties in eastern Tennessee. Samples of symptomatic black walnuts were sent to the University of Tennessee for processing. The causative agents, walnut twig beetle, Pityophthorus juglandis, and the fungal pathogen Geosmithia morbida, were documented on the same trees in four counties. Tree mortality was observed in two counties, and tree decline was observed in at least 10 counties although it may be attributed to previous droughts or to TCD. In 2010, four confirmed counties were quarantined by TDA, and 10 buffer counties were also regulated. Research is underway to further assess the incidence and impact of TCD on black walnut in Tennessee.

Geosmithia morbida sp. nov., a new phytopathogenic species living in symbiosis with the walnut twig beetle (Pityophthorus juglandis) on Juglans in USA

Widespread morbidity and mortality of Juglans nigra has occurred in the western USA over the past decade. Tree mortality is the result of aggressive feeding by the walnut twig beetle (Pityophthorus juglandis) and subsequent canker development around beetle galleries caused by a filamentous ascomycete in genus Geosmithia (Ascomycota: Hypocreales). Thirty-seven Geosmithia strains collected from J. californica, J. hindsii, J. major and J. nigra in seven USA states (AZ, CA, CO, ID, OR, UT, WA) were compared with morphological and molecular methods (ITS rDNA sequences). Strains had common characteristics including yellowish conidia en masse, growth at 37 C and absence of growth on Czapek-Dox agar and belonged to a single species described here as G. morbida. Whereas Geosmithia are common saprobes associated with bark beetles attacking hardwoods and conifers worldwide, G. morbida is the first species documented as a plant pathogen.

Thousand Cankers Disease is Widespread in Black Walnut in the Western United States

Thousand cankers disease of black walnut is caused by aggressive feeding by the walnut twig beetle and subsequent canker development around beetle galleries caused the fungus Geosmithia morbida. The authors We confirmed the presence of G. morbida from symptomatic black walnut or hybrids in California, Colorado, Idaho, New Mexico, Oregon, Utah, and Washington. Thousand cankers disease continues to cause extensive mortality to black walnut over a wide geographic region and is intensifying in the western United States. Accepted for publication 2 June 2011. Published 30 June 2011.

POTENTIAL IMPACTS OF THOUSAND CANKERS DISEASE ON COMMERCIAL WALNUT PRODUCTION AND WALNUT GERMPLASM CONSERVATION

Thousand cankers disease (TCD) is a recently identified insect/fungal disease complex responsible for mortality of eastern black walnut trees, Juglans nigra, planted in non-native areas of the western United States. The presence in California of both the insect vector, the walnut twig beetle, Pityophthorus juglandis, and the fungal pathogen, a species of Geosmithia, raises concerns regarding the potential impact of the disease on commercial walnut production and the long term health of other walnut species native to California and American Southwest. Movement of the disease eastward could threaten native J. nigra populations in hardwood forests of the eastern United States. Identification of both the vector and the fungal pathogen of TCD from trees in the USDA Germplasm Collection near Davis, CA may impact ex situ germplasm management.

Black Walnut Mortality in Colorado Caused by the Walnut Twig Beetle and Thousand Cankers Disease

Since 2001, widespread mortality of black walnut (Juglans nigra) has been reported in Colorado, USA. Affected trees initially show a yellowing and thinning of leaves in the upper crown, followed by twig and branch dieback and ultimately tree death. We report that this mortality is the result of a combination of an expanded geographic range of the walnut twig beetle (Pityophthorus juglandis), its aggressive feeding behavior on black walnut, and extensive cankering caused by an unnamed Geosmithia fungus associated with the beetle. Geosmithia was consistently recovered from the bodies of P. juglandis and this insect introduces the fungus into healthy trees during gallery formation. This is the first report of Geosmithia as a pathogen of black walnut. We propose the name Thousand Cankers to describe this disease because mortality is the result of bark necrosis caused by an enormous number of coalescing branch and trunk cankers. A second pathogen, Fusarium solani, was isolated from the margins of elongate trunk cankers during the final stages of decline, but not from cankers surrounding beetle galleries. Thousand Cankers Disease is eliminating black walnut along the Front Range of Colorado and poses a grave risk to this species in its native range in eastern North America should the insect/Geosmithia complex be introduced. Accepted for publication 26 May 2009. Published 11 August 2009.

The role of Pityophthorus spp. as vectors of pitch canker affecting Pinus radiata

AbstractThe fungus Fusarium circinatum Nirenberg and O’Donnell (Hypocreaceae) causes pitch canker, a disease affecting pines worldwide. In California, many native insect species have been implicated in transmission of F. circinatum. This study showed that two twig beetle species, Pityophthorus setosus Blackman and Pityophthorus carmeli Swaine (Coleoptera: Curculionidae: Corthylini), can make wounds on healthy Monterey pine (Pinus radiata D. Don (Pinaceae)) branches that are suitable for infection by the pitch canker pathogen. Because these two species are not known to engage in maturation feeding and the observed wounds were not associated with tunneling, we hypothesize that the wounds reflect “exploratory tasting” to assess the suitability of the substrate for colonization. This behavior would help to explain how twig beetles can serve as wounding agents on healthy host branches, which are not amenable to colonization by these insects. We tested two specific hypotheses: (1) two native species of Pityophthorus can create wounds on F. circinatum-contaminated trees that are sufficient for development of disease; and (2) the efficiency with which F. circinatum infects beetle wounds is affected by relative humidity. Under growth-chamber conditions, both Pityophthorus species indulged in exploratory behavior that caused wounds suitable for development of pitch canker. Field experiments did not confirm a significant effect of beetle activity on infection frequency, perhaps because of an overall low infection rate due to low temperatures. Experiments conducted under controlled conditions documented a significant effect of relative humidity on the success rate of twig beetle-initiated infections.

Notes on the Occurrence of Nemosoma attenuatum Van Dyke, 1915 (Coleoptera: Trogossitidae), in California with a Literature Review and Museum Survey of Nemosoma spp

The bark gnawing beetle, Nemosoma attenuatum Van Dyke (Coleoptera: Trogossitidae), has been previously described as an associate of twig beetles in the genus Pityophthorus Eichhoff (Coleoptera: Curculionidae) and other bark beetles. Presented here are post hoc observations on the seasonal occurrence of N. attenuatum, collected and identified as part of an ecological study of Pityophthorus species inhabiting native Monterey pines and their potential role as vectors of a fungal pathogen Fusarium circinatum. Adult N. attenuatum were recovered from flight traps between May 2001 and July 2003. Additionally, adult and larval N. attenuatum were discovered in cut P. radiata branches suspended in canopies of P. radiata in native stands. The highest flight trap counts were recorded during May 2001, suggesting a mass emergence. The highest numbers of N. attenuatum were observed from branches suspended between January to March. Two N. attenuatum larvae were discovered within the galleries of Pityophthorus setosus, although neither was observed feeding on any Pityophthorus spp. Neither eggs nor pupae were discovered during the course of this study. Also included are a literature review and the results of a museum survey of five major North American insect collections for Nemosoma species from North America.

Response of Pityophthorus puberulus (Coleoptera: Curculionidae: Scolytinae) to pheromones and terpenes

AbstractIn field studies conducted to improve trapping programs for the white pine cone beetle, Conophthorus coniperda (Schwarz), a twig beetle, Pityophthorus puberulus (LeConte), a common associate of eastern white pine, Pinus strobus L. (Pinaceae), in seed orchards, was also captured. In these experiments, P. puberulus was attracted to traps containing the sex pheromone pityol with or without the monoterpenes (−)-α-pinene and (−)-β-pinene, but attraction to traps was inhibited when S-(−)-limonene was present. We speculate that limonene may play an important role in host recognition and suitability, by indicating a resource suitable for C. coniperda but unsuitable for P. puberulus.

Colonization of cut branches of five coniferous hosts of the pitch canker fungus by Pityophthorus spp. (Coleoptera: Scolytidae) in central, coastal California

AbstractPitch canker of pines (Pinus spp.) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) (Pinaceae) is caused by the fungus Fusarium circinatum Nirenberg et O'Donnell. In California, infections by F. circinatum occur largely through wounds caused by insects. Field experiments were initiated to determine whether the colonization activities of twig beetles, Pityophthorus spp. (Coleoptera: Scolytidae), could explain the incidence of pitch canker on Monterey pine (P. radiata D. Don), Bishop pine (P. muricata D. Don), ponderosa pine (P. ponderosa var. ponderosa Dougl.), knobcone pine (P. attenuata Lemm.), and Douglas-fir. Asymptomatic branches were cut from each of four pairs of tree species (Monterey–Bishop, Monterey–ponderosa, Monterey–knobcone, Monterey–Douglas-fir) at four sites and attached to the lower canopy of both heterospecific and conspecific host trees (total of four combinations per pair). After 10 weeks, branches were collected and placed in rearing tubes in the laboratory. Emerging insects were identified and placed on a Fusarium-selective medium. Monterey, Bishop, and ponderosa pines were more heavily infested by Pityophthorus spp. than Douglas-fir and knobcone pine. Furthermore, more Pityophthorus beetles emerged from Monterey pine branches placed in Monterey pine canopies than from Monterey pine branches placed in Bishop or ponderosa pine canopies, indicating that reduced emergence (colonization) was caused by the hetero specific host. Relatively fewer insects emerged from sites containing either Monterey and knobcone pines or Monterey pine and Douglas-fir. Fusarium circinatum was not isolated from emerging Pityophthorus spp. Susceptibility of the five host species, based on mean lesion lengths resulting from mechanical inoculations, varied significantly. The longest lesions were on Monterey pine and the shortest were on ponderosa pine and Douglas-fir. The low incidence of pitch canker on Douglas-fir and ponderosa pine in nature compared with that on Monterey, Bishop, and knobcone pines may be explained by the low colonization by twig beetles and the greater resistance of Douglas-fir and ponderosa pine to this disease, compared with the other three hosts.

Twig beetles, Pityophthorus spp. (Coleoptera: Scolytidae), as vectors of the pitch canker pathogen in California

AbstractTwig beetles in the genus Pityophthorus Eichhoff are known to be associated with the pitch canker pathogen, Fusarium circinatum, in California. Phoresy of the pathogen on these species has been reported to occur when insects emerge from diseased branches and when they infest disease-free, cut branch tips. To demonstrate that twig beetles can vector the pathogen, studies of phoresy and transmission were conducted in a native Monterey pine, Pinus radiata D. Don (Pinaceae), forest. Phoresy was confirmed for both Pityophthorus setosus Blackman and Pityophthorus carmeli Swaine, and P. setosus was shown to vector the pitch canker pathogen when contaminated with fungal spores and caged onto Monterey pine branches. When attractive baits were used to increase visitation to Monterey pines by P. setosus, baited trees were more likely to develop pitch canker than unbaited trees even though the beetles did not tunnel into the host to develop egg galleries. Therefore, twig beetles are competent as vectors of the pitch canker pathogen, and their vectoring activity, though requiring a wound, does not require that they establish egg galleries in the host.